(FORTUNE Magazine) – If you care about nutrition, a little siren probably goes off in your head when somebody passes the salt. Medical experts have drummed into us for more than 40 years that eating too much salt causes high blood pressure, making us have heart attacks and strokes. Since 1980 the federal government has joined in, pushing lower salt for all.

All along, skeptics have disagreed in vain. But a flurry of findings over the past two years seems to be blowing away the condiment's bad rap. A few months ago, ABC-TV even blasted common wisdom on salt as "junk science."

Welcome to the latest round in the international salt war. It has always been a down and dirty food fight. In 1978, Mother Jones magazine hurled this headline at food producers: "Salt: The Deadly White Powder You Already Snort." Striking back in a health magazine, a prominent doctor dripped acid on the campaign to lower salt consumption: "Now, what can I possibly have against these well-intentioned efforts? Nothing except the fact that ... they are based on assumptions that are either entirely false or entirely unproven!" The war has become so heated recently that one of the scientists involved raised the possibility of a libel claim against a distinguished medical journal.

Unlike many dietary disputes, the salt war is far from a case of academic trivial pursuit. Not only does the food industry have a huge stake in its outcome, but so do we, given all the salt we eat and its metabolic importance.

The average American consumes between nine and ten grams of salt, or sodium chloride, each day; the government recommends we eat no more than six grams, about a teaspoon. We crave the stuff for good reasons--without sodium, our nerves couldn't transmit electrical impulses, our muscles would go limp, and our inner blood-pressure controls would fail. So important is salt that our bodies are downright miserly about it and can stretch a tiny amount a long way: Our average intake is ten to 100 times the minimum needed to get by.

In our diets, salt is everywhere. Ten percent of the salt we eat is what we sprinkle on at the table; 15% we add in cooking. The rest, some 75%, comes in processed food, everything from apple pie to frozen dinners. This so-called nondiscretionary salt is a food manufacturer's best friend. Cut back too much on the amount of salt, and hot dogs, for instance, turn to mush (see box). Flavor collapses too. Says Rhona Applebaum of the National Food Processors Association in Washington, D.C.: "Salt enhances all flavors and its absence is keenly noticed in many foods." If consumers ever get religious about sodium, as many have about fat, sales of perennial favorites could get clobbered: canned soups, processed meats, frozen pizza, to name a few.

So far, Americans are more intent on cutting fat than cutting salt. About 23% of consumers say salt in food is a "serious" health risk, vs. 44% for fat, according to the Food Marketing Institute in Washington, D.C. That could change--the salt war is about diseases of aging, a subject of growing urgency to baby-boomers, who are notorious for embracing health fads. Excess salt in the diet has been linked to osteoporosis, kidney stones, and other illnesses.

High blood pressure is the main worry, however. Some 50 million U.S. adults, more than one in four, suffer from it, according to the government's best estimate. There's no doubt hypertension increases the risk of strokes, heart disease, and other cardiovascular scourges--even "high normal" blood pressure, between 130/85 and 140/90 millimeters of mercury--has been found to double stroke risk. Researchers recently found that hypertension literally makes the brain shrink with age, dimming the wits.

Our blood pressure usually rises as we age, and by 60, about half of us have edged over 140/90. Doctors consider that high enough to treat with drugs, but only one in four cases of hypertension is detected and controlled--unlike our growing girths, high blood pressure often doesn't advertise itself until we keel over.

If cut-the-salt crusaders are right, everyday sodium-laced food is a major contributor to this insidious rise--significantly, they note, blood pressure doesn't rise with age among "preliterate" people who eat little salt, such as the Yanomamo Indians of South America. Thus, salt may qualify as one of the few truly huge health hazards, comparable to cholesterol and smoking. Eating less of it could add quality time to tens of millions of lives.

Skeptics have never bought that. They concede that a low-salt diet can reduce blood pressure in some hypertensives. But they insist that urging everyone to cut back on salt goes too far. Says Dr. Michael Alderman, president of the American Society of Hypertension and an iconoclast on salt: "After all this time, there's still no evidence that people who eat less sodium will live a longer or better life."

This "not proven" argument is the skeptics' mantra--they've been saying it for years. Until recently, however, most salt data provided ammunition for the anti-salt forces. For instance, studies suggest some 25% of people with normal blood pressure and 50% of hypertensives are "salt sensitive"--their blood pressure shoots up after a hefty dose of salt. Before you sink your teeth into your next Reuben sandwich, consider: How do you know you're not one of the tens of millions of Americans at risk from salt? You could get tested for salt sensitivity--one test involves monitoring a person's blood pressure while giving him four hours of intravenous saline solution, followed by a course of salt-lowering drugs. It's hardly feasible to test everybody; thus, the government's decision to urge everyone to cut back on salt arguably makes sense.

Trouble for the antisalt crusaders has come in the form of two studies published in the past two years. One, led by Alderman at the Albert Einstein College of Medicine in the Bronx, N.Y., studied some 3,000 hypertension patients whose treatment included advice to avoid high-salt foods. The researchers found a surprising correlation: Men gauged to be eating less salt at the start of the multiyear study had more heart attacks later on.

The second study was funded by Campbell Soup Co. and carried out by researchers at the University of Toronto, who pooled and analyzed data from 56 previous studies on salt and blood pressure. As reported in the Journal of the American Medical Association, the researchers concluded that when people without hypertension cut their sodium intake, the drop in their blood pressure is "insignificant."

Together, the studies show that recommending lower salt for all is "premature" at best and may even cause harm, argues Dr. Alexander Logan, co-author of the Toronto study. The Salt Institute, an Alexandria, Va., trade group representing salt manufacturers worldwide, couldn't agree more. Soon after the studies appeared, it cited them in a petition to the Food and Drug Administration to revoke a regulation allowing food sellers to claim health benefits for low-salt products.

After decades of bad news about sodium, the new data offered a classic man-bites-dog story, and the media gleefully paraded the pro-salt studies. salt is regaining favor and savor declared a New York Times headline last year. Says Bonnie Liebman, director of nutrition at the Center for Science in the Public Interest, a Washington, D.C., advocacy group known for pushing food processors to add less salt: "I hate to say it, but I think the Salt Institute and food industry have gained ground in convincing the public that salt doesn't matter."

To Dr. Jeremiah Stamler, a grizzled veteran of the salt war, the commotion is much ado about little: "If commercial interests weren't trying to torpedo the idea that people would be healthier eating less salt, there would be no debate about it. It's clear that multiple factors affect blood pressure. But it doesn't follow that we should forget about salt." A 77-year-old professor of preventive medicine at Northwestern University, Stamler has co-authored hundreds of studies on cardiovascular risk factors since the 1940s. "The totality of evidence," he avers, "is overwhelming that a high sodium intake raises blood pressure."

Stamler asserts that Alderman's study--the one linking low salt with greater risk of heart attacks--says little if anything about normal people who reduce salt, for it involved hypertensive patients on drugs who suddenly cut their sodium intake to a level far below the government's guideline. And the women in the study who ate less salt didn't have more heart attacks. Many other studies of people who eat less salt have failed to find adverse effects.

As for the Toronto study, it actually showed that hypertensives who cut down on sodium lowered their blood pressure by a sizable amount--a fact downplayed in its conclusions. The Toronto team also committed what some scientists feel is a glaring sin of omission: It failed to report data it had collected on normal volunteers who ate low-salt meals in hospitals and clinics. Since such subjects' salt intake was carefully controlled, they may well have shown larger blood pressure reductions than people who ate at home--a result that would have undercut the not-to-worry message on salt. Co-author Logan counters that the omission was justified, since "institutionally fed people are not in the real world."

Logan's industry ties have gotten the most heat, though. He asserts he recently was slandered at a medical meeting by a colleague who publicly insinuated that Logan had concealed one of his industry connections. The Campbell funding was duly noted in the study. But Logan also advises the Salt Institute, a tie not widely known before it was publicized at the meeting in an accusatory way. Logan counters that he had disclosed the link in a written statement to the meeting's organizers.

Fighting back, Logan recently hinted he might slap the Lancet, a British medical journal, with a libel suit after one of its writers sent him a draft of a story spotlighting the medical-meeting flap. "She portrayed me as a liar," claims Logan. "It was like something in the National Enquirer." The story, which hasn't yet appeared, has since been "toned way down," he says. Comments Lancet editor Richard Horton: "It was surprising and hugely disappointing to me that someone of Dr. Logan's scientific stature should feel compelled to call his lawyer rather than engage in proper scientific debate."

In another behind-the-scenes slugfest, the Salt Institute has demanded through its attorneys that Stamler, the antisalt senior statesman, hand over data that he and colleagues compiled in the 1980s for the largest salt study ever. Known as Intersalt, it examined the correlation between blood pressure and sodium intake in more than 10,000 people in 32 countries. Intersalt was expected to deliver a heavy blow to the skeptics. Instead it turned out to be what one researcher calls a "Rorschach test"--what it says about salt depends on who's sifting its data.

Would-be sifters must first get the data, and Stamler balked in 1994 when the Salt Institute asked for Intersalt's raw numbers. At issue is Intersalt's most controversial finding: The more salt a person eats, the more his blood pressure rises as he ages. "That sticks in industry's craw," says Stamler. The two sides eventually worked out a partial release of the data, which the trade group fashioned into an "Intersalt revisited" analysis--it showed no age-related rise. It appeared in the British Medical Journal last year, along with a disclaimer from the editors and rebuttals from Stamler and others that called it incompetent, opaque, obfuscatory, erroneous, confusing, and bizarre, not to mention fatally flawed.

Intersalt shows why the salt war continues to rage: While the study did link higher sodium and hypertension, the strength of the connection is hotly debated. Many things contribute to high blood pressure--stress, smoking, obesity--and disentangling salt's effect is devilishly tricky. Statistical adjustments are needed to correct for all these "confounders," and that opens the door to endless feuds about whether data have been cooked.

With Intersalt clouded by statistical bickering, low-salt advocates eagerly awaited a report, published in March, called "The Trials of Hypertension Prevention," a major study aimed at establishing that low-salt diets prevent high blood pressure. But its subjects didn't cut their salt intake long enough to show compelling benefits. John LaRosa, chancellor of Tulane Medical Center in New Orleans, says the same problem slowed research on cholesterol: "With dietary interventions, we got levels down by 5% to 15%, but it wasn't enough to show major benefits. Once good cholesterol-lowering drugs were developed, we could get levels down by 30%, and that showed very dramatic effects."

That is the main reason the salt debate may never end: It's almost impossible to get lots of people to cut way back for years, which is the only definitive way to test the pros and cons of eating less. It's too easy to fall off the wagon: Studies show that after you cut back by 40% or 50%, as the government recommends, your sense of taste readjusts after about two months--you'll find yourself liking salt less. But dine out a few times on salty restaurant food or come home tired and microwave a pizza, and you'll find yourself craving the stuff again: Salt breeds a taste for salt, which revives much faster than it fades.

One study led by an Australian named Derek Denton stands out like a beacon. Published two years ago in the journal Nature Medicine, it electrified scientists on both sides of the war.

Denton, director of the University of Melbourne's Howard Florey Institute, is one of the biggest guns in salt science: When the U.S. National Academy of Sciences named him a foreign associate a few years ago, it described him as "the world's leading authority" on salt metabolism.

His study involved a colony of 26 captive chimpanzees in Gabon that had long been on low-salt vegetarian diets. Over 22 weeks, Denton's team ramped up the salt consumption of half the colony to roughly as much as Westerners eat daily, then kept the chimps at that level for nearly two years. The other chimps' diets weren't changed. Importantly, the researchers carefully held constant everything except salt, which made it possible to isolate salt's effects with unprecedented clarity--even the chimps' stress levels were constant, since their social hierarchy didn't change.

The results: The blood pressures of 70% of the high-salt chimps rose in tandem with their sodium levels. What's more, their pressures rapidly fell to "baseline" when low-salt diets were resumed. "I don't think the rise was due to alcohol, smoking, or stress," says Denton.

The study immediately came under fire. In its FDA petition, the Salt Institute asserted, among other things, that the high-salt chimps had been deprived of water, making their blood pressure rise. That's not only false, says Denton, "it's quite silly. The study clearly notes that the animals were producing two to four liters of urine a day--more than most of us excrete."

The critics' main charge is that chimps aren't human, so the study is irrelevant. But as far as blood pressure and salt metabolism go, chimps look precisely like small, muscular people--not surprising, given that 98.4% of their DNA is identical to ours. Besides, adds Denton, rural, "preliterate" human groups have been studied whose traditional hunter-gatherer diets are similar to the Gabon chimps' normal fare; for all but one of 39 such groups, members who move to cities and are exposed to high-salt diets have shown the same pattern of rapid blood pressure rise. (The exception is a set of Caribbean families whose members seem genetically resistant to salt.)

But what about the Japanese, who traditionally eat loads of salt, yet live long? Denton notes that they have significantly cut down on salt over the past two decades--before that, inland Japanese farmers who ate the most salt had one of the world's highest rates of hypertension and stroke, and seldom lived past 70. He adds that Japan's low-fat diet may offset some of salt's ill effects.

In his chimp report, Denton canvasses the scientific literature on salt and shows how it fits into an evolutionary big picture. He notes that most of our prehistoric ancestors' diets were probably very low in salt, a rarity in food before salt mines were developed. Because of sodium's metabolic importance, evolution geared us to like the taste of it in order to ensure we get enough. Now that salt's everywhere, we eat it to excess--just as we do fat, another dietary rarity in the bad old days. Gorging on salt from infancy onward may help set our metabolic and blood-vessel controls so that blood pressure rises with age.

Denton concedes much of this is speculation. Yet it explains a great deal. Example: Human milk is low in sodium compared with that of most mammals, for prehistoric moms couldn't physiologically afford to give lots of sodium to infants over the many months of nursing. Thus, infants had to be metabolically set for low salt. In any case, says Denton, critics who would dismiss his theory out of hand must first take on a man who has proved very hard to beat: Charles Darwin.